Improve end-of-buffer speed.
Add `GOAMD64_v3` version with small improvement for matching.
For now guarded by build tags to not duplicate all code.
```
benchmark old ns/op new ns/op delta
BenchmarkTwainEncode1e1/default-32 8.32 8.28 -0.49%
BenchmarkTwainEncode1e1/better-32 8.36 8.32 -0.53%
BenchmarkTwainEncode1e1/best-32 8.31 8.32 +0.02%
BenchmarkTwainEncode1e1/snappy-default-32 8.34 8.32 -0.17%
BenchmarkTwainEncode1e1/snappy-better-32 8.31 8.31 +0.00%
BenchmarkTwainEncode1e1/snappy-best-32 8.31 8.29 -0.20%
BenchmarkTwainEncode1e1/snappy-ref-noasm-32 7.61 7.62 +0.22%
BenchmarkTwainEncode1e2/default-32 94.4 93.8 -0.70%
BenchmarkTwainEncode1e2/better-32 273 269 -1.36%
BenchmarkTwainEncode1e2/best-32 76827 75007 -2.37%
BenchmarkTwainEncode1e2/snappy-default-32 94.7 93.6 -1.17%
BenchmarkTwainEncode1e2/snappy-better-32 273 268 -1.58%
BenchmarkTwainEncode1e2/snappy-best-32 72735 72867 +0.18%
BenchmarkTwainEncode1e2/snappy-ref-noasm-32 471 469 -0.25%
BenchmarkTwainEncode1e3/default-32 872 867 -0.62%
BenchmarkTwainEncode1e3/better-32 2416 2403 -0.54%
BenchmarkTwainEncode1e3/best-32 128772 128589 -0.14%
BenchmarkTwainEncode1e3/snappy-default-32 869 862 -0.84%
BenchmarkTwainEncode1e3/snappy-better-32 2415 2402 -0.54%
BenchmarkTwainEncode1e3/snappy-best-32 94544 92615 -2.04%
BenchmarkTwainEncode1e3/snappy-ref-noasm-32 2317 2328 +0.47%
BenchmarkTwainEncode1e4/default-32 10080 9862 -2.16%
BenchmarkTwainEncode1e4/better-32 24173 23778 -1.63%
BenchmarkTwainEncode1e4/best-32 638221 632676 -0.87%
BenchmarkTwainEncode1e4/snappy-default-32 10038 9900 -1.37%
BenchmarkTwainEncode1e4/snappy-better-32 24088 23655 -1.80%
BenchmarkTwainEncode1e4/snappy-best-32 336750 334551 -0.65%
BenchmarkTwainEncode1e4/snappy-ref-noasm-32 25050 24941 -0.44%
BenchmarkTwainEncode1e5/default-32 208338 204080 -2.04%
BenchmarkTwainEncode1e5/better-32 400069 382699 -4.34%
BenchmarkTwainEncode1e5/best-32 5249363 5374492 +2.38%
BenchmarkTwainEncode1e5/snappy-default-32 207783 200382 -3.56%
BenchmarkTwainEncode1e5/snappy-better-32 388589 378026 -2.72%
BenchmarkTwainEncode1e5/snappy-best-32 2889378 2781338 -3.74%
BenchmarkTwainEncode1e5/snappy-ref-noasm-32 487332 484808 -0.52%
BenchmarkTwainEncode1e6/default-32 2305542 2251826 -2.33%
BenchmarkTwainEncode1e6/better-32 4023332 3904791 -2.95%
BenchmarkTwainEncode1e6/best-32 53576955 54518800 +1.76%
BenchmarkTwainEncode1e6/snappy-default-32 2300992 2179567 -5.28%
BenchmarkTwainEncode1e6/snappy-better-32 3938222 3879487 -1.49%
BenchmarkTwainEncode1e6/snappy-best-32 30057235 30808837 +2.50%
BenchmarkTwainEncode1e6/snappy-ref-noasm-32 4890432 4866709 -0.49%
BenchmarkTwainEncode1e7/default-32 23717990 22395276 -5.58%
BenchmarkTwainEncode1e7/better-32 42845300 42508469 -0.79%
BenchmarkTwainEncode1e7/best-32 1113607500 1111374800 -0.20%
BenchmarkTwainEncode1e7/snappy-default-32 23335686 22315622 -4.37%
BenchmarkTwainEncode1e7/snappy-better-32 42227550 41652074 -1.36%
BenchmarkTwainEncode1e7/snappy-best-32 410723367 421980100 +2.74%
BenchmarkTwainEncode1e7/snappy-ref-noasm-32 51418814 51197981 -0.43%
```